Why can't it be 'becws'?
It could be, but popty or siop fara are more commonly used in general. We stick with the more common Welsh and English words and expressions in order to keep things manageable.
"Becws" is pretty widely used up here.
OK, we now have [siop fara/becws/popty] = [baker's/bread shop/bakery] everywhere, all done lovingly by hand!